1. Field of the Invention
The present invention pertains to improved means for the measurement of noise characteristics of oscillators and more particularly to means for the measurement of frequency modulation noise on the carrier output of low-power microwave signal sources such as those employing a crystal oscillator and a frequency multiplier.
2. Description of the Prior Art
Means for the measurement of noise signals generated in high frequency oscillators are known in the art. Systems are known, for instance, that employ high frequency discriminators wherein the noise measurement threshold is determined by noise internally generated within crystal diode detectors in a discriminator device. Such systems, however, provide a limited output power so that measurement of the frequency modulation noise characteristics is not practical for evaluating such sources as high frequency crystal oscillator or semiconductor signal sources.
U.S. Pat. No. 3,675,124 issued in the names of the present inventors and assigned to the present assignee, concerns a system for the measurement of the frequency modulation noise on carrier signals from low power, high frequency sources such as cavity stabilized semiconductor diode oscillators. An auxiliary injection phase locked oscillator driven by the oscillator to be tested, provides an output signal in which the noise spectrum of the oscillator to be tested is reproduced at an amplitude sufficient to drive a frequency discriminator at a level permitting noise measurement. Modulation characteristics of crystal oscillators and frequency multiplier signal sources, however, produce FM noise which may overload this system.
Although the aforementioned noise measurement systems have proven useful in most instances, they are of limited value when measurements on certain state-of-the-art frequency synthesizer sources are required. The problem becomes particularly acute when measuring the FM noise of a microwave signal source which is phase synchronized to a multiple of high frequency or very high frequency crystal controlled oscillator wherein noise frequency deviation is small in the low baseband region but rises rapidly as the baseband region increases. The deviation in the higher baseband region may overdrive the wave analyzer and mask the frequency deviation which is the subject of the measurement. The present invention is directed principally to overcoming this problem.